1. Field of the Invention
This invention relates to a process for the production of improved polyurethane casting resins.
Polyurethanes have for years been used in many branches of industry: foams, lacquers and adhesives produced from polyurethanes are widely used. Another increasingly important field of application for polyurethanes is that of cast-molded solid or rigid resins. One reason for the increasing use of solid polyurethanes is the fact that they are easily shaped as required by a number of processes: for example, polyurethane elastomers may be extruded, calendered and injection-molded. Vulcanizable, rubber-like polyurethane mixtures are formable by any of the forming techniques commonly used in the rubber industry, and, in addition, solid polyurethane moldings are producible by machine-based or manual casting processes. In this latter field, polyurethane materials compete technologically with unsaturated polyesters, epoxy resins and cold-crosslinking silicone rubbers. By comparison with such other resin systems, especially cold-hardening epoxy resins, polyurethane casting-resin systems are of interest because they are easier to process and are relatively inexpensive raw materials. One important field of application for these systems is in electrical insulation where cast-molded polyurethane materials are much-used as cable sealing compounds, battery seals, cover pottings for capacitors, or as embedding compounds for bell transformers.
2. Discussion of Related Art
Urethane casting resin systems are widely known from the prior art. Broadly, the systems comprise a reactive isocyanate (a compound containing at least two terminal isocyanate (N=C=O) groups, herein termed a "polyisocyanate") and a chain extender for the isocyanate, generally termed a "hardener." These two components are homogeneously mixed, optionally in the presence of a catalyst and customary additives such as fillers, pigments, or drying agents. The resulting mixture is then cast into the mold, and hardened by the polyaddition reaction of isocyanate and hardener to form the corresponding urethane polymer, with the result dependent upon the components used, the hardener used, the quantity of catalyst used, and other reaction parameters necessary to the formation of the polymer. Polyurethane casting systems of this type are well-known, and are described, for example, in G. W. Becker and D. Braun Kunststoffhandbuch, Vol. VII (Polyurethane), Verlag Hanser, (1983); or in B. A. Dombrow, Polyurethanes, 2nd Edition, New York (1965).
Polyether polyols, polyester polyols and polyether polyester polyols are generally employed as chain extenders for the production of polyurethane casting resins. These prior art polyols are typically synthesized from petroleum rather than from renewable raw materials capable of regeneration from natural sources. In view of the increasing shortage of petroleum resources, therefore, there is a need for hardeners for the production of urethane casting resins which are based on regenerable natural sources.
One such natural product which has been proposed as a hardener for the production of urethane casting resins is castor oil. Castor oil was used long ago as a hardener for polyurethane potting compounds for the potting of electronic components ("Plastics Laboratory", Princeton University, "Castor Oil Polyurethanes and Applications as Potting Components", U.S.O.T. page 99165). However, the solid polyurethane materials obtained using castor oil as hardener did not conform to modern requirements with respect to tensile strength, hardness, resistance to distortion under heat, and electrical properties.